The Question:

Who said or wrote: "If I have seen further it is by standing on
the shoulders of giants"?

The Answer:

It was written—or at least quoted—by a man often considered the
greatest scientist ever, Sir Isaac Newton.

Newton's views on optics were met by a lot of
criticism when they were first presented in the late 1600s. His
discovery that white light was made up of small particles of differing
sizes that could be split into colors of the rainbow by a prism was
quite unique and his views were in direct conflict with many
scientists of the day.

He modestly attributed his discoveries to the work done by his
predecessors. Newton explained in a letter that, if he had seen
further than other men, "it is by standing on the shoulders of
giants."

British scholars, however, have traced the phrase "standing on
the shoulders of giants" back to the 12th century and philosopher
Bernard of Chartres.